Thirty-four women with normal menstrual cycles but suffering from severe premenstrual mastalgia were randomly treated with either tamoxifen (10 mg daily) (18 women) or placebo (16 women) from cycle day 5 to 24 for six consecutive cycles. At the end of the treatment, 89% of the tamoxifen-treated patients were free from the symptoms and the remainder experienced partial alleviation. In contrast, six patients treated with placebo showed only partial alleviation (38%) (p less than 0.001). Twelve months after the end of tamoxifen treatment, 53% of the medicated women were still free of symptoms as compared with none of the placebo-treated patients. These results suggest that tamoxifen is highly effective for the management of severe premenstrual mastalgia and should be useful for the treatment of this disorder.